1. Field of the Invention
This invention relates to an iron-based mixed powder for powder metallurgy. More particularly, this invention is suitable for obtaining a sintered compact having excellent machinability, but is not limited to this application.
2. Description of the Related Art
Progress in powder metallurgical techniques has enabled the production of parts having complicated shapes and high dimensional precision near to xe2x80x9cnet shapexe2x80x9d (requiring substantially no machining for obtaining a target shape). An iron-based powder metallurgical product is generally produced by mixing an iron-based powder, a powder for an alloy such as a copper powder, a graphite powder, or the like, and a lubricant such as zinc stearate, lithium stearate or the like to form an iron-based mixed powder for powder metallurgy, filling a die(mold) with the resultant mixed powder, pressing the mixture under pressure, sintering the green compact to form a sintered compact and, if required, machining the product. The thus-produced sintered compact has a high content of voids and, thus, has higher cutting force than metallic materials obtained by a solution process. Therefore, to improve the machinability of the sintered compact, any one of various powders of Pb, Se, Te, S, MnS, BaS, CaS, and the like or an alloy thereof is conventionally added to the iron-based mixed powder.
However, Pb has a melting point as low as 330xc2x0 C. and, thus, has the problem that it is melted in the sintering process and is not dissolved in iron, thereby causing difficulties in homogeneous dispersing Pb in a base matrix. Se and Te embrittle the sintered compact and, thus, have the problem of significantly deteriorating the mechanical properties of the sintered compact.
Japanese Examined Patent Publication No. 46-39564 discloses a free-cutting metallic material produced by a powder metallurgical method in which BaSO4 or BaS is added to iron or an iron-based alloy singly or in a mixture. This publication also discloses that the method improves machinability by adding BaSO4 or BaS singly or in a mixture. Japanese Examined Patent Publication No. 52-16684 discloses a method of producing sintered steel with excellent machinability in which a mixed powder obtained by adding calcium sulfide CaS or calcium sulfate CaSO4 to an iron-based raw material powder is pressed and then sintered.
However, mixing with S or a S-containing compound such as MnS or the like as a machinability improving powder causes the problem of contaminating the refractory of the sintering furnace, the conveyor mesh belt, heating device, etc. with H2S produced in sintering, thereby decreasing the lifetime of these parts. In addition, there is also the problem of deteriorating the appearance of the sintered compact. Therefore, mixing a S-containing compound as a machinability improving powder with the iron-based mixed powder is avoided. Furthermore, when BaS, CaS, or the like remains in the sintered compact, the sintered compact has the problem in which corrosion easily occurs due to hygroscopicity of BaS and CaS.
For these problems, for example, Japanese Unexamined Patent Publication No. 57-198201 discloses a steel powder for sintering which contains 0.001 to 0.10% of Ca and 0.05 to 1.0% of O and which provides a sintered compact having good machinability. The sintered compact produced by using the sintering powder disclosed in Japanese Unexamined Patent Publication No. 57-198201 does not have the problem of contaminating the sintering furnace because S is not contained, but the sintered compact has the problem of deteriorating fluidity of the powder and destabilizing pressing because calcium oxide has hygroscopicity.
Japanese Unexamined Patent Publication No. 7-507358 discloses an iron-based powder composition containing 0.1 to 0.6% by weight of calcium fluoride CaF2 and having improved machinability. However, according to the findings obtained in research conducted by the inventors, the method of simply mixing calcium fluoride in a free state as disclosed in this publication cannot satisfactorily improve machinability. Furthermore, impurities contained in calcium fluoride CaF2 might affect the dimensional changes and mechanical properties of the sintered compact. Therefore, this method is preferably carried out with caution, such as using high-purity calcium fluoride.
Japanese Unexamined Patent Publication No.9-279204 discloses an iron-based mixed powder for powder metallurgy which contains 0.02 to 0.3% by weight of CaOxe2x80x94Al2O3xe2x80x94SiO2 system compound oxide powder mainly comprising an iron powder and having an anorthite phase and/or gehlenite phase and an average particle diameter of 50 xcexcm or less. However, unless the CaOxe2x80x94Al2O3xe2x80x94SiO2 system compound oxide powder containing fewer impurities and having a limited particle size is used, there is the problem of deteriorating the properties of the powder and the sintered compact.
Japanese Unexamined Patent Publication No. 63-137137 discloses a method of producing sintered steel in which a graphite powder comprising an alkali earth fluoride in an amount corresponding to 0.1 to 1.2% by weight of a raw material iron powder, the alkali earth fluoride being partially or entirely adhered to the surfaces of the graphite powder, is added to the raw material iron powder, and the resultant mixture is sintered. This publication also discloses that the sintered steel produced by the method has excellent machinability. However, the technique disclosed in Japanese Unexamined Patent Publication No. 63-137137 must use an alkali earth fluoride ground to a small particle size of about {fraction (1/10)} of the particle size of the graphite powder, thereby causing the problem of adding the step of grinding the alkali earth fluoride as a separate step and significantly increasing the production cost.
It would accordingly be advantageous to solve the above problems of conventional techniques and to provide an iron-based mixed powder capable of improving machinability of a sintered compact without deteriorating the mechanical properties of the sintered compact and a sintering furnace.
We intensively studied machinability improving powders capable of improving machinability without deteriorating the mechanical properties of a sintered compact. As a result, it was found that a phosphate compound of an alkali earth metal, particularly, calcium phosphate compound is effective. As a result of further experiment and research based on that finding, this invention was achieved.
In accordance with a first aspect of the invention, an iron-based mixed powder for powder metallurgy comprises an iron-based powder, an alloy powder (a powder for an alloy), a machinability improving powder, and a lubricant, wherein the machinability improving powder contains a phosphate compound of an alkali earth metal.
The invention also provides an iron-based sintered compact obtained by pressing the iron-based mixed powder of the invention resulting in a green compact, and then sintering the green compact.
The phosphate compound of an alkali earth metal comprises preferably calcium phosphate compound, and more preferably hydroxyapatite. The machinability improving powder preferably substantially consists of calcium phosphate compound (more preferably, hydroxyapatite) with substantially no additive intentionally added. The hydroxyapatite preferably comprises crystallites of over about 200 xc3x85, preferably over about 600 xc3x85.
In the first aspect of the invention, the machinability improving powder preferably further comprises an alkali earth fluoride, preferably calcium fluoride. In this case, the machinability improving powder preferably comprises calcium phosphate compound and calcium fluoride, and more preferably comprises hydroxyapatite preferably comprising crystallites of over about 200 xc3x85, more preferably over about 600 xc3x85, and calcium fluoride. In this case, the machinability improving powder preferably comprises calcium fluoride and calcium phosphate compound at a ratio (content of calcium fluoride)/(content of calcium phosphate compound) of about 0.8 or more in terms of Ca.
The machinability improving powder more preferably substantially consists of calcium phosphate compound and calcium fluoride, or hydroxyapatite comprising crystallites of over about 200 xc3x85, preferably over about 600 xc3x85, and calcium fluoride, with substantially no additive intentionally added.
Particularly, when a compound containing Ca is mainly used for the machinability improving powder, the machinability improving powder is preferably contained in a total amount of about 0.02 to about 0.39% by mass in terms of Ca based on the total amount of the iron-based powder, for the alloy powder and the machinability improving powder.
In the first aspect of the invention, calcium phosphate compound is preferably at least one selected from tricalcium phosphate, calcium monohydrogen phosphate, calcium dihydrogen phosphate, and hydroxyapatite.
In the first aspect of the invention, the content of the alloy powder is preferably about 5% by mass or less based on the total amount of the iron-based powder, the alloy powder and the machinability improving powder.
In the first aspect of the invention, the content of the lubricant is preferably about 0.2 to about 1.5 parts by weight based on the total amount of 100 parts by weight of the iron-based powder, the alloy powder and the machinability improving powder.
In the first aspect of the invention, the alloy powder and/or the machinability improving powder is preferably adhered to the surfaces of a part or the whole of the iron-based powder.
We intensively studied the influences of various factors on the machinability of the sintered compact. As a result, we arrived at the conclusion that from the viewpoint of improving the appearance of the sintered compact and prevention of contamination of the sintering furnace, an alkali earth metal fluoride powder (i.e., powdered) is also effective as the machinability improving powder. We further found that by using the alkali earth metal fluoride powder as the machinability improving powder, the machinability of the sintered compact is significantly improved by fixing the machinability improving powder, together with a graphite powder, to the surfaces of the iron-based powder with a binder. This is because when the iron-based mixed powder comprising the graphite powder and the machinability improving powder, which are fixed to the surfaces thereof, is pressed and then sintered to form the sintered compact, the graphite powder and the alkali earth metal fluoride powder can be put into direct contact with each other in a so-called xe2x80x9cdewaxedxe2x80x9d state in which the lubricant and the binder are substantially evaporated in the sintering process, thereby significantly improving the machinability of the sintered compact.
In a second aspect of the invention, an iron-based mixed powder for powder metallurgy comprises an iron-based powder, an alloy powder including a graphite powder, a machinability improving powder, a binder, and a lubricant, wherein the machinability improving powder contains an alkali earth metal fluoride powder, and the graphite powder and the alkali earth metal fluoride powder are fixed to the surfaces of the iron-based powder, preferably to concave portion on the surface of the iron-based powder, with the binder.
The invention also provides an iron-based sintered compact obtained by pressing the above iron-based mixed powder resulting in a green compact, and then sintering the green compact.
In the invention, the machinability improving powder more preferably substantially consists of the alkali earth metal fluoride powder with substantially no additive intentionally added.
The alkali earth metal fluoride powder preferably comprises at least one of calcium fluoride, magnesium fluoride, strontium fluoride, and barium fluoride.
In the second aspect of the invention, the machinability improving powder is preferably contained at a content of about 0.1 to about 0.7% by mass based on the total amount of the iron-based powder, the alloy powder, and the machinability improving powder.
In the second aspect of the invention, the lubricant is preferably a free lubricant in a free state.
In the invention, the content of the alloy powder is preferably about 0.5 to about 7% by mass based on the total amount of the iron-based powder, the alloy powder, and the machinability improving powder. In the invention, the alloy powder preferably comprises a graphite powder or further comprises a metal powder and/or an alloyed metal powder. The content of the graphite powder is preferably about 0.5 to about 7% by mass, and more preferably about 0.5 to about 5% by mass, based on the total amount of the iron-based powder, the alloy powder, and the machinability improving powder.
In the invention, the content of the lubricant is preferably about 0.1 to about 0.5 part by mass based on the total amount of 100 parts by weight of the iron-based powder, the alloy powder, and the machinability improving powder.
In the invention, the content of the binder is preferably about 0.1 to about 1.0 part by mass based on the total amount of 100 parts by mass of the iron-based powder, the alloy powder, and the machinability improving powder.